Quandry / Query - NOT Porsche related ... need abs help

[Cortina]

This one has me "confused".
Car : circa 2006 Skoda Fabia - 100k miles
ABS warning light comes on (inital stationary check) . (Long story in middle here - which involves diagnostics , new wiring , new sensor , etc etc). Jack up the car - (JUST-ie nothing else !) , recheck ... light goes out.

No logic. Take off wheel , set car down (either on disc or on suspension arm) light out , put wheel back on .... up in air , light out . Remove sensor from hub ... plug in , check , light goes out. Put wheel back on , set down as normal ... warning light comes on.

This is not possible we say ... where is logic (check brake fluid level - rats , that's ok too) .... fault remains.

WHAT am I missing ?
Local garage thought pick up ring in bearing faulty - hmm , if that was the case , why does light go out when sensor is removed ??
Have given up for now - mot in 2 hours time , too late to fix .... but where to look ?

any ideas welcome !
cheers
Alan

[Bruce M]

Getting a fault code for the sensor I assume? Nothing else ABS related the cause?

The ECU can only detect a short or broken wire statically. If the sensor isn’t either of those, then the fault is only detected while the car is moving because the pulsing signal will not be plausible when compared to the other wheels.

[Cortina]

Not sure I fully understand you. I am NOT sure what the fault code is at the "fail" point. The abs warning light simply comes on . Car is not moving - therefore it can only be sensing wiring or whatever it picks up at a standstill. The wire going to a front sensor was "iffy" - when disassembled , and therefore replaced. Fault simply repeated under same conditions - changing sensor ... repeat.
So what can possibly change from jacked up to not jacked up ... in terms of wiring ? (Very little from my perspective - hence baffled !)

A full scan of system on garage machine indicated intermittent on a certain corner - the one we were fiddling with.

hmmm

[Nine One One]

Does the garage have VCDS (Vag Com) if not try and find someone local that has that system that can come and plug it into the car.

VW Group Group cars are very temperamental on battery voltage, and can throw up codes when something is replaced if the battery is low.

1) Did they clear the original fault code?

2) Is it an original sensor, there are some shite Chinese copies out there.

3) If you jack the car up, and rotate the wheel can you see it reading a speed or signal on the scan tool - if not the ABS/Wheel sensor is no good

4) Did the sensor come out OK? Did you need to use a hammer or other tool to exert force on it. I only ask as there is a lot of crud and rust that can get attached to the reluctor ring. If you look down the hole where the sensor fits does it look clean, if not clean it with brake cleaner on a cloth with a screwdriver to hold the cloth in place.

5) If it is coming on, when the wheel is under load, wheel bearing causing contact against the reluctor ring? Have you tried putting on the ramp, say 3 feet in the air, wheel off, use the disc brake with a trolley jack underneath to put load on it and see at what point the light goes off? (same as when you put it down on the ground with tyre on but now you can see when it goes out). When it does go out, wiggle the wires and see if the light comes on at any point.

6) With the scan tool have run an ABS check to see if the pump is actually working and cycle through that check? You may have fixed the ABS sensor and it may now be causing another fault.

My thoughts are either the new sensor is no good, the ABS pump, or the wiring that has been replaced…………also check the battery voltage.

Also I think there are two fuses for the ABS system on a Fabia one in the battery box area and another in the normal fuse panel, check those as well. You could have blown one when replacing the wiring?

[Cortina]

Thanks for the response - I'll copy and send on to owner (BIL) and see what that brings. We don't have a tool that will talk to the car , so that makes things awkward. The fact that light clears with sensor not in hub , makes me shy away from bearing / ring related issues. WIP
cheers
Alan

[KS]

An ABS fault can be triggered (and stored) if tyre pressures are uneven - a tyre with lower pressure is smaller in diameter than another with the correct pressure, so the wheel turns more often over a given distance. This tricks the ABS monitoring system into believing one wheel is a locking-up, or on the verge thereof. I went through this with my Golf just before I sold it.

[Cortina]

An ABS fault can be triggered (and stored) if tyre pressures are uneven - a tyre with lower pressure is smaller in diameter than another with the correct pressure, so the wheel turns more often over a given distance. This tricks the ABS monitoring system into believing one wheel is a locking-up, or on the verge thereof. I went through this with my Golf just before I sold it.

Keith - I would agree with you on a semi modern car (like my up GTI) , I would not be so sure on a nearly 20 year old Skoda Fabia.
Ta for the response. Car failed MOT last night (as expected - on ABS light) ... now in testing / head scratching / checking mode.
Cheers
Alan

[Bruce M]

I use a cheap xtool V401 for VAG cars. It can read all the car modules for the full set of fault code. Plus reset inspection light, reset airbag codes & trigger ABS bleeding.

However it is not user friendly & i had to mess about doing a firmware update (it has a removable SD card so the firmware can be updated). The new firmware was too big for the supplied SD card so I had to use a bigger SD card.

[Cortina]

OK - Issue found , sorted and mot redone !
Faulty wiring (too many Birthdays) on the Rear ABS sensor - so that was both sensors and both bits of wiring on the same side (front and rear) ! What are the odds of that ?.
Problem coming and going by jacking up the front - was "caused" by the fact that when the front was jacked up - the rear also moved (obviously by a much lesser amount .... but enough to cause a problem !)
Interesting ... and another school day
Thanks for all the help - it did help !

cheers
Alan